High temperature mixing

ABSTRACT

A catalyst comprising a mixed metal oxide is useful for the vapor phase oxidation of an alkane or a mixture of an alkane and an alkene to an unsaturated carboxylic acid and for the vapor phase ammoxidation of an alkane or a mixture of an alkane and an alkene to an unsaturated nitrile.

[0001] The present invention relates to an improved process for thevapor phase catalytic oxidation of alkanes, or a mixture of alkanes andalkenes, to their corresponding unsaturated carboxylic acids.

[0002] The present invention also relates to a process for the vaporphase catalytic oxidation of alkanes, or a mixture of alkanes andalkenes, in the presence of ammonia, to their corresponding unsaturatednitrites.

[0003] Moreover, the present invention relates to a process forproducing an improved catalyst comprising a mixed metal oxide and thecatalyst so-formed.

[0004] Nitriles, such as acrylonitrile and methacrylonitrile, have beenindustrially produced as important intermediates for the preparation offibers, synthetic resins, synthetic rubbers, and the like. The mostpopular method for producing such nitrites is to subject an olefin suchas propene or isobutene to a gas phase catalytic reaction with ammoniaand oxygen in the presence of a catalyst at a high temperature. Knowncatalysts for conducting this reaction include a Mo—Bi—P—O catalyst, aV—Sb—O catalyst, an Sb—U—V—Ni—O catalyst, a Sb—Sn—O catalyst, aV—Sb—W—P—O catalyst and a catalyst obtained by mechanically mixing aV—Sb—W—O oxide and a Bi—Ce—Mo—W—O oxide. However, in view of the pricedifference between propane and propene or between isobutane andisobutene, attention has been drawn to the development of a method forproducing acrylonitrile or methacrylonitrile by an ammoxidation reactionwherein a lower alkane, such as propane or isobutane, is used as astarting material, and it is catalytically reacted with ammonia andoxygen in a gaseous phase in the presence of a catalyst.

[0005] In particular, U.S. Pat. No. 5,281,745 discloses a method forproducing an unsaturated nitrile comprising subjecting an alkane andammonia in the gaseous state to catalytic oxidation in the presence of acatalyst which satisfies the conditions:

[0006] (1) the mixed metal oxide catalyst is represented by theempirical formula

Mo_(a)V_(b)Te_(c)X_(x)O_(n)

[0007] wherein X is at least one element selected from the groupconsisting of niobium, tantalum, tungsten, titanium, aluminum,zirconium, chromium, manganese, iron, ruthenium, cobalt, rhodium,nickel, palladium, platinum, antimony, bismuth, boron and cerium and,when a=1, b=0.01 to 1.0, c=0.01 to 1.0, x=0.01 to 1.0 and n is a numbersuch that the total valency of the metal elements is satisfied; and

[0008] (2) the catalyst has X-ray diffraction peaks at the followingangles (±0.3°) of 2θ in its X-ray diffraction pattern: 22.1°, 28.2°,36.2°, 45.2° and 50.0°.

[0009] Similarly, Japanese Laid-Open Patent Application Publication No.6-228073 discloses a method of nitrile preparation comprising reactingan alkane in a gas phase contact reaction with ammonia in the presenceof a mixed metal oxide catalyst of the formula

W_(a)V_(b)Te_(c)X_(x)O_(n)

[0010] wherein X represents one or more elements selected from niobium,tantalum, titanium, aluminum, zirconium, chromium, manganese, iron,ruthenium, cobalt, rhodium, nickel, palladium, platinum, antimony,bismuth, indium and cerium and, when a=1, b=0.01 to 1.0, c=0.01 to 1.0,x=0.01 to 1.0 and n is determined by the oxide form of the elements.

[0011] Unsaturated carboxylic acids such as acrylic acid and methacrylicacid are industrially important as starting materials for varioussynthetic resins, coating materials and plasticizers. Commercially, thecurrent process for acrylic acid manufacture involves a two-stepcatalytic oxidation reaction starting with a propene feed. In the firststage, propene is converted to acrolein over a modified bismuthmolybdate catalyst. In the second stage, acrolein product from the firststage is converted to acrylic acid using a catalyst composed of mainlymolybdenum and vanadium oxides. In most cases, the catalyst formulationsare proprietary to the catalyst supplier, but, the technology is wellestablished. Moreover, there is an incentive to develop a single stepprocess to prepare the unsaturated acid from its corresponding alkene.Therefore, the prior art describes cases where complex metal oxidecatalysts are utilized for the preparation of unsaturated acid from acorresponding alkene in a single step.

[0012] Commercial incentives also exist for producing acrylic acid usinga lower cost propane feed. Therefore, the prior art describes caseswherein a mixed metal oxide catalyst is used to convert propane toacrylic acid in one step.

[0013] U.S. Pat. No. 5,380,933 discloses a method for producing anunsaturated carboxylic acid comprising subjecting an alkane to a vaporphase catalytic oxidation reaction in the presence of a catalystcontaining a mixed metal oxide comprising, as essential components, Mo,V, Te, O and X, wherein X is at least one element selected from thegroup consisting of niobium, tantalum, tungsten, titanium, aluminum,zirconium, chromium, manganese, iron, ruthenium, cobalt, rhodium,nickel, palladium, platinum, antimony, bismuth, boron, indium andcerium; and wherein the proportions of the respective essentialcomponents, based on the total amount of the essential components,exclusive of oxygen, satisfy the following relationships:

0.25<r(Mo)<0.98, 0.003<r(V)<0.5, 0.003<r(Te)<0.5 and 0.003<r(X)<0.5,

[0014] wherein r(Mo), r(V), r(Te) and r(X) are the molar fractions ofMo, V, Te and X, respectively, based on the total amount of theessential components exclusive of oxygen.

[0015] U.S. Pat. No. 5,994,580 discloses a process for producing acrylicacid from propane and oxygen gas through a vapor-phase catalyticoxidation reaction, the process comprising conducting the reaction usinga metal oxide catalyst containing the elements Mo, V, Sb and A (whereinA is at least one element selected from the group consisting of Nb, Ta,Sn, W, Ti, Ni, Fe, Cr and Co).

[0016] Japanese Laid-Open Patent Application Publication No. 2000-037623discloses a method for producing an unsaturated carboxylic acidcomprising subjecting an alkane to a vapor phase catalytic oxidation inthe presence of a catalyst having the empirical formula

MoV_(a)Nb_(b)X_(c)Z_(d)O_(n)

[0017] wherein X is at least one element selected from the groupconsisting of Te and Sb, Z is at least one element selected from thegroup consisting of W, Cr, Ta, Ti, Zr, Hf, Mn, Re, Fe, Ru, Co, Rh, Ni,Pd, Pt, Ag, Zn, B, Al, Ga, In, Ge, Sn, Pb, P, Bi, Y, rare earth elementsand alkaline earth elements, 0.1≦a≦1.0, 0.01≦b≦1.0, 0.01≦c≦1.0, 0≦d≦1.0and n is determined by the oxidation states of the other elements.

[0018] Despite the above-noted attempts to provide new and improvedcatalysts for the oxidation of alkanes to unsaturated carboxylic acids,one impediment to the provision of a commercially viable process forsuch catalytic oxidations is the identification of a catalyst providingadequate conversion and suitable selectivity, thereby providingsufficient yield of the unsaturated product.

[0019] By the present invention, there is provided a catalyst whereinthe activity and/or selectivity is enhanced and, hence, the overallyield of the desired reaction product is also enhanced.

[0020] Thus, in a first embodiment, the present invention provides aprocess for producing an improved catalyst comprising a mixed metaloxide having the empirical formula

Mo_(a)V_(b)N_(c)X_(d)Z_(e)O_(f)

[0021] wherein N is at least one element selected from the groupconsisting of Te and Sb,

[0022] wherein X is at least one element selected from the groupconsisting of Nb, Ta, Ti, W, Se, Al, Zr, Cr, Mn, Fe, Ru, Co, Rh, Ni, Pt,Bi, B, In, As, Ge, Sn, Li, Na, K, Rb, Cs, Fr, Be, Mg, Ca, Sr, Ba, Ra,Hf, Pb, P, Pm, Eu, Gd, Dy, Ho, Er, Tm, Yb and Lu,

[0023] wherein Z is at least one element selected from the groupconsisting of Au, Ag, Re, Pr, Zn, Ga, Pd, Ir, Nd, Y, Sm, Tb, Br, Cu, Sc,Cl, F and I, and

[0024] wherein, when a=1, b=0.01 to 1.0, c=0.01 to 1.0, d=0.01 to 1.0,e=0 to 0.1 and f is dependent on the oxidation state of the otherelements, comprising:

[0025] (a) admixing compounds of elements Mo, V, N, X and Z and at leastone solvent at a temperature greater than or equal to 45° C. to form anadmixture,

[0026] wherein Mo, V, N, X and Z are present in such amounts that theatomic ratio of Mo:V:N:X:Z is a:b:c:d:e, and, when a=1, b=0.01 to 1.0,c=0.1 to 1.0, d=0.01 to 1.0 and e=0 to 0.1;

[0027] (b) removing said at least one solvent from the so-formedadmixture to form a catalyst precursor; and

[0028] (c) calcining said catalyst precursor to obtain said mixed metaloxide.

[0029] In a second embodiment, the present invention provides animproved catalyst prepared by the process of the first embodiment.

[0030] In a third embodiment, the present invention provides a processfor producing an unsaturated carboxylic acid which comprises subjectingan alkane, or a mixture of an alkane and an alkene, to a vapor phasecatalytic oxidation reaction in the presence of a catalyst comprising amixed metal oxide having the empirical formula

Mo_(a)V_(b)N_(c)X_(d)Z_(e)O_(f)

[0031] wherein N is at least one element selected from the groupconsisting of Te and Sb,

[0032] wherein X is at least one element selected from the groupconsisting of Nb, Ta, Ti, W, Se, Al, Zr, Cr, Mn, Fe, Ru, Co, Rh, Ni, Pt,Bi, B, In, As, Ge, Sn, Li, Na, K, Rb, Cs, Fr, Be, Mg, Ca, Sr, Ba, Ra,Hf, Pb, P, Pm, Eu, Gd, Dy, Ho, Er, Tm, Yb and Lu,

[0033] wherein Z is at least one element selected from the groupconsisting of Au, Ag, Re, Pr, Zn, Ga, Pd, Ir, Nd, Y, Sm, Tb, Br, Cu, Sc,Cl, F and I, and

[0034] wherein, when a=1, b=0.01 to 1.0, c=0.01 to 1.0, d=0.01 to 1.0,e=0 to 0.1 and f is dependent on the oxidation state of the otherelements, prepared by the process comprising:

[0035] (a) admixing compounds of elements Mo, V, N, X and Z and at leastone solvent at a temperature greater than or equal to 45° C. to form anadmixture,

[0036] wherein Mo, V, N, X and Z are present in such amounts that theatomic ratio of Mo:V:N:X:Z is a:b:c:d:e, and, when a=1, b=0.01 to 1.0,c=0.1 to 1.0, d=0.01 to 1.0 and e=0 to 0.1;

[0037] (b) removing said at least one solvent from the so-formedadmixture to form a catalyst precursor; and

[0038] (c) calcining said catalyst precursor to obtain said mixed metaloxide.

[0039] In a fourth embodiment, the present invention provides a processfor producing an unsaturated nitrile which comprises subjecting analkane, or a mixture of an alkane and an alkene, and ammonia to a vaporphase catalytic oxidation reaction in the presence of a catalystcomprising a mixed metal oxide having the empirical formula

Mo_(a)V_(b)N_(c)X_(d)Z_(e)O_(f)

[0040] wherein N is at least one element selected from the groupconsisting of Te and Sb,

[0041] wherein X is at least one element selected from the groupconsisting of Nb, Ta, Ti, W, Se, Al, Zr, Cr, Mn, Fe, Ru, Co, Rh, Ni, Pt,Bi, B, In, As, Ge, Sn, Li, Na, K, Rb, Cs, Fr, Be, Mg, Ca, Sr, Ba, Ra,Hf, Pb, P, Pm, Eu, Gd, Dy, Ho, Er, Tm, Yb and Lu,

[0042] wherein Z is at least one element selected from the groupconsisting of Au, Ag, Re, Pr, Zn, Ga, Pd, Ir, Nd, Y, Sm, Tb, Br, Cu, Sc,Cl, F and I, and

[0043] wherein, when a=1, b=0.01 to 1.0, c=0.01 to 1.0, d=0.01 to 1.0,e=0 to 0.1 and f is dependent on the oxidation state of the otherelements, prepared by the process comprising:

[0044] (a) admixing compounds of elements Mo, V, N, X and Z and at leastone solvent at a temperature greater than or equal to 45° C. to form anadmixture,

[0045] wherein Mo, V, N, X and Z are present in such amounts that theatomic ratio of Mo:V:N:X:Z is a:b:c:d:e, and, when a=1, b=0.01 to 1.0,c=0.1 to 1.0, d=0.01 to 1.0 and e=0 to 0.1;

[0046] (b) removing said at least one solvent from the so-formedadmixture to form a catalyst precursor; and

[0047] (c) calcining said catalyst precursor to obtain said mixed metaloxide.

[0048] The mixed metal oxide to be used as a catalyst component of thepresent invention has the empirical formula

Mo_(a)V_(b)N_(c)X_(d)Z_(e)O_(f)

[0049] wherein N is at least one element selected from the groupconsisting of Te and Sb,

[0050] wherein X is at least one element selected from the groupconsisting of Nb, Ta, Ti, W, Se, Al, Zr, Cr, Mn, Fe, Ru, Co, Rh, Ni, Pt,Bi, B, In, As, Ge, Sn, Li, Na, K, Rb, Cs, Fr, Be, Mg, Ca, Sr, Ba Hf, Pb,P, Pm, Eu, Gd, Dy, Ho, Er, Tm, Yb and Lu,

[0051] wherein Z is at least one element selected from the groupconsisting of Au, Ag, Re, Pr, Zn, Ga, Pd, Ir, Nd, Y, Sm, Tb, Br, Cu, Sc,Cl, F and I, and

[0052] wherein, when a=1, b=0.01 to 1.0, c=0.01 to 1.0, d=0.01 to 1.0,e=0 to 0.1 and f is dependent on the oxidation state of the otherelements.

[0053] Preferably, when a=1, b=0.1 to 0.5, c=0.05 to 0.5, d=0.01 to 0.5and e=0.001 to 0.1. More preferably, when a=1, b=0.15 to 0.45, c=0.05 to0.45, d=0.01 to 0.1 and e=0.001 to 0.1. The value of f, i.e. the amountof oxygen present, is dependent on the oxidation state of the otherelements in the catalyst. However, f is typically in the range of from 3to 4.7.

[0054] Preferred promoted mixed metal oxides have the empirical formulaeMo_(a)V_(b)Te_(c)Nb_(d)Z_(e)O_(f) and Mo_(a)V_(b)Sb_(c)Nb_(d)Z_(e)O_(f)wherein Z, a, b, c, d, e and f are as previously defined.

[0055] The mixed metal oxide can be prepared in the following manner.

[0056] In a first step an admixture, i.e. a slurry or a solution, isformed by admixing metal compounds, preferably at least one of whichcontains oxygen, and at least one solvent, at a temperature greater thanor equal to 45° C., preferably greater than or equal to 50° C., morepreferably greater than or equal to 60° C., in appropriate amounts toform the slurry or solution. The metal compounds contain elements Mo, V,N, X and Z, as previously defined. The admixing of the metal compoundsand the at least one solvent at a temperature greater than or equal to45° C. means that upon completion of the addition of all the componentsto the admixture, the admixture is at a temperature of greater than orequal to 45° C. For example, each ingredient added to the admixture maybe at a temperature greater than or equal to 45° C. Alternatively, eachingredient may be at a temperature greater than or less than 60° C. solong as, when all of the ingredients are added together, the resultanttemperature of the admixture is greater than or equal to 60° C. There isno particular upper limit to the temperature at which the admixture isformed, but, for ease of handling, 100° C. can be used as a practicablelimit.

[0057] Suitable solvents include water; alcohols including, but notlimited to, methanol, ethanol, propanol, and diols, etc.; as well asother polar solvents known in the art. Generally, water is preferred.The water is any water suitable for use in chemical syntheses including,without limitation, distilled water and de-ionized water. The amount ofwater present is preferably an amount sufficient to keep the elementssubstantially in solution long enough to avoid or minimize compositionaland/or phase segregation during the preparation steps. Accordingly, theamount of water will vary according to the amounts and solubilities ofthe materials combined.

[0058] For example, when a mixed metal oxide of the formulaMo_(a)V_(b)Te_(c)Nb_(d)O_(f), wherein the element N is Te and theelement X is Nb, is to be prepared, an aqueous solution of niobiumoxalate may be added to an aqueous solution or slurry of ammoniumheptamolybdate, ammonium metavanadate and telluric acid, so that theatomic ratio of the respective metal elements would be in the prescribedproportions.

[0059] Once the admixture, i.e. the aqueous slurry or solution, isformed, the water is removed by any suitable method, known in the art,to form a catalyst precursor. Such methods include, without limitation,vacuum drying, freeze drying, spray drying, rotary evaporation and airdrying. Vacuum drying is generally performed at pressures ranging from10 mmHg to 500 mmHg. Freeze drying typically entails freezing the slurryor solution, using, for instance, liquid nitrogen, and drying the frozenslurry or solution under vacuum. Spray drying is generally performedunder an inert atmosphere such as nitrogen or argon, with an inlettemperature ranging from 125° C. to 200° C. and an outlet temperatureranging from 75° C. to 150° C. Rotary evaporation is generally performedat a bath temperature of from 25° C. to 90° C. and at a pressure of from10 mmHg to 760 mmHg, preferably at a bath temperature of from 40° to 90°C. and at a pressure of from 10 mmHg to 350 mmHg, more preferably at abath temperature of from 40° C. to 60° C. and at a pressure of from 10mmHg to 40 mmHg. Air drying may be effected at temperatures ranging from25° C. to 90° C. Rotary evaporation is generally utilized.

[0060] Once obtained, the catalyst precursor is calcined. Thecalcination may be conducted in an oxygen-containing atmosphere or inthe substantial absence of oxygen, e.g., in an inert atmosphere or invacuo. The inert atmosphere may be any material which is substantiallyinert, i.e., does not react or interact with, the catalyst precursor.Suitable examples include, without limitation, nitrogen, argon, xenon,helium or mixtures thereof. Preferably, the inert atmosphere is argon ornitrogen. The inert atmosphere may flow over the surface of the catalystprecursor or may not flow thereover (a static environment). When theinert atmosphere does flow over the surface of the catalyst precursor,the flow rate can vary over a wide range, e.g., at a space velocity offrom 1 to 500 hr⁻¹.

[0061] The calcination is usually performed at a temperature of from350° C. to 850° C., preferably from 400° C. to 700° C., more preferablyfrom 500° C. to 640° C. The calcination is performed for an amount oftime suitable to form the aforementioned catalyst. Typically, thecalcination is performed for from 0.5 to 30 hours, preferably from 1 to25 hours, more preferably for from 1 to 15 hours, to obtain the desiredpromoted mixed metal oxide.

[0062] In a preferred mode of operation, the catalyst precursor iscalcined in two stages. In the first stage, the catalyst precursor iscalcined in an oxidizing environment (e.g. air), e.g., at a temperatureof from 200° C. to 400° C., preferably from 275° C. to 325° C. for from15 minutes to 8 hours, preferably for from 1 to 3 hours. In the secondstage, the material from the first stage is calcined in a non-oxidizingenvironment (e.g., an inert atmosphere), e.g., at a temperature of from500° C. to 750° C., preferably for from 550° C. to 650° C., for 15minutes to 8 hours, preferably for from 1 to 3 hours. Optionally, areducing gas, such as, for example, ammonia or hydrogen, may be addedduring the second stage calcination.

[0063] In a particularly preferred mode of operation, the catalystprecursor in the first stage is placed in the desired oxidizingatmosphere at room temperature and then raised to the first stagecalcination temperature and held there for the desired first stagecalcination time. The atmosphere is then replaced with the desirednon-oxidizing atmosphere for the second stage calcination, thetemperature is raised to the desired second stage calcinationtemperature and held there for the desired second stage calcinationtime.

[0064] Although any type of heating mechanism, e.g., a furnace, may beutilized during the calcination, it is preferred to conduct thecalcination under a flow of the designated gaseous environment.Therefore, it is advantageous to conduct the calcination in a bed withcontinuous flow of the desired gas(es) through the bed of solid catalystprecursor particles.

[0065] With calcination, a catalyst is formed having the formulaMO_(a)V_(b)N_(c)X_(d)Z_(e)O_(f) wherein N, X, Z, a, b, c, d, e and f areas previously defined.

[0066] The starting materials for the above promoted mixed metal oxideare not limited to those described above. A wide range of materialsincluding, for example, oxides, nitrates, halides or oxyhalides,alkoxides, acetylacetonates, and organometallic compounds may be used.For example, ammonium heptamolybdate may be utilized for the source ofmolybdenum in the catalyst. However, compounds such as MoO₃, MoO₂,MoCl₅, MoOCl₄, Mo(OC₂H₅)₅, molybdenum acetylacetonate, phosphomolybdicacid and silicomolybdic acid may also be utilized instead of ammoniumheptamolybdate. Similarly, ammonium metavanadate may be utilized for thesource of vanadium in the catalyst. However, compounds such as V₂O₅,V₂O₃, VOCl₃, VCl₄, VO(OC₂H₅)₃, vanadyl sulfate, vanadium acetylacetonateand vanadyl acetylacetonate may also be utilized instead of ammoniummetavanadate. The tellurium source may include telluric acid, TeCl₄,Te(OC₂H₅)₅, Te(OCH(CH₃)₂)₄ and TeO₂. The niobium source may includeammonium niobium oxalate, Nb₂O₅, NbCl₅, niobic acid or Nb(OC₂H₅)₅ aswell as the more conventional niobium oxalate.

[0067] A mixed metal oxide, thus obtained, exhibits excellent catalyticactivities by itself. However, the mixed metal oxide may be converted toa catalyst having higher activities by contact with a liquid contactmember selected from the group consisting of organic acids, alcohols,inorganic acids and hydrogen peroxide.

[0068] Contacting with a liquid contact member selected from the groupconsisting of organic acids, alcohols, inorganic acids and hydrogenperoxide may be effected without any particular restrictions. In thisregard, the liquid contact member is normally used in an amount of 1 to100 times the volume of the mixed metal oxide or the first calcinedprecursor, preferably 3 to 50 times the volume, more preferably 5 to 25times the volume. Contacting at elevated temperatures is permissible,however, if prolonged contact time is not a consideration, contacting atroom temperature may be utilized. Normally, contact temperatures of roomtemperature to 100° C. are utilized, preferably 50° C. to 90° C., morepreferably 60° C. to 80° C. As previously noted, contact time will beaffected by the temperature at which the contacting is carried out.Normally, contact times of 1 to 100 hours are utilized, preferably 2 to20 hours, more preferably 5 to 10 hours. The contact mixture ispreferably agitated during the contacting.

[0069] There are no particular restrictions upon the organic acids whichmay be used as the liquid contacting member. For example, oxalic acid,formic acid, acetic acid, citric acid and tartaric acid may be used,however, oxalic acid is preferred. If the organic acid is a liquid, itmay be used as is or in an aqueous solution. If the organic acid is asolid, it is used in an aqueous solution. When using aqueous solutions,there are no particular restrictions on the concentration of the organicacid. Normally, the concentration of the organic acid in the aqueoussolution can vary from 0.1 to 50% by weight, preferably 1 to 15% byweight.

[0070] There are no particular restrictions upon the alcohols which maybe used as the liquid contacting member. For example, methanol, ethanol,propanol, butanol, hexanol and diols may be utilized, however, alcoholshaving one to four carbon atoms are preferred, with ethylene glycolbeing particularly preferred. The alcohols may be utilized in the formof aqueous solutions, but, if so, the water content should be held to20% by weight or less for the best effectiveness.

[0071] Similarly, there are no particular restrictions upon theinorganic acids which may be used as the liquid contacting member. Forexample, telluric acid, nitric acid, sulfuric acid, phosphoric acid,hydrochloric acid, perchloric acid, chloric acid and hypochlorous acidmay be used. The inorganic acids are typically used as aqueous solutionswith concentrations of the acids in the range of from 0.1 to 50% byweight, preferably from 0.1 to 10% by weight.

[0072] When hydrogen peroxide is utilized as the liquid contactingmember, it is used in the form of an aqueous solution having aconcentration in the range of from 0.1 to 50% by weight, preferably from1 to 10% by weight.

[0073] After contacting with the liquid contacting member, insolublematerial is recovered from the so-formed contact mixture. The insolublematerial may be recovered by any conventional method, e.g.,centrifugation or filtration. If the contacting was conducted atelevated temperature, the contact mixture may be cooled prior torecovery of the insoluble material. If desired, the recovered insolublematerial may be washed with one of the previously disclosed solvents,preferably water. Moreover, the recovered insoluble material may berecalcined. If such recalcination is effected, it is preferred that itbe carried out under an inert atmosphere.

[0074] Additionally, the mixed metal oxide may be converted to acatalyst having higher activities by grinding.

[0075] There is no particular restriction as to the grinding method, andconventional methods may be employed. As a dry grinding method, a methodof using a gas stream grinder may, for example, be mentioned whereincoarse particles are permitted to collide with one another in a highspeed gas stream for grinding. The grinding may be conducted not onlymechanically but also by using a mortar or the like in the case of asmall scale operation.

[0076] As a wet grinding method wherein grinding is conducted in a wetstate by adding water or an organic solvent to the above mixed metaloxide, a conventional method of using a rotary cylinder-type medium millor a medium-stirring type mill, may be mentioned. The rotarycylinder-type medium mill is a wet mill of the type wherein a containerfor the object to be ground is rotated, and it includes, for example, aball mill and a rod mill. The medium-stirring type mill is a wet mill ofthe type wherein the object to be ground, contained in a container isstirred by a stirring apparatus, and it includes, for example, a rotaryscrew type mill, and a rotary disc type mill.

[0077] The conditions for grinding may suitably be set to meet thenature of the above-mentioned promoted mixed metal oxide, the viscosity,the concentration, etc. of the solvent used in the case of wet grinding,or the optimum conditions of the grinding apparatus.

[0078] Further, in some cases, it is possible to further improve thecatalytic activities by further adding a solvent to the ground catalystprecursor to form a solution or slurry, followed by drying again. Thereis no particular restriction as to the concentration of the solution orslurry, and it is usual to adjust the solution or slurry so that thetotal amount of the starting material compounds for the ground catalystprecursor is from 10 to 60wt %. Then, this solution or slurry is driedby a method such as spray drying, freeze drying, evaporation to drynessor vacuum drying, preferably by the spray drying method. Further,similar drying may be conducted also in the case where wet grinding isconducted.

[0079] The oxide obtained by the above-mentioned method may be used as afinal catalyst, but it may further be subjected to heat treatmentusually at a temperature of from 200° to 700° C. for from 0.1 to 10hours.

[0080] The mixed metal oxide thus obtained may be used by itself as asolid catalyst, but may be formed into a catalyst together with asuitable carrier such as silica, alumina, titania, aluminosilicate,diatomaceous earth or zirconia. Further, it may be molded into asuitable shape and particle size depending upon the scale or system ofthe reactor.

[0081] Alternatively, the metal components of the presently contemplatedcatalyst may be supported on materials such as alumina, silica,silica-alumina, zirconia, titania, etc. by conventional incipientwetness techniques. In one typical method, solutions containing themetals are contacted with the dry support such that the support iswetted; then, the resultant wetted material is dried, for example, at atemperature from room temperature to 200° C. followed by calcination asdescribed above. In another method, metal solutions are contacted withthe support, typically in volume ratios of greater than 3:1 (metalsolution:support), and the solution agitated such that the metal ionsare ion-exchanged onto the support. The metal-containing support is thendried and calcined as detailed above. In any event, the admixture of thecomponents, i.e. metal compounds and solvent(s), must produce anadmixture having a temperature greater than or equal to 45° C.

[0082] As previously noted, the present invention provides a process forproducing an unsaturated carboxylic acid, which comprises subjecting analkane, or a mixture of an alkane and an alkene, to a vapor phasecatalytic oxidation reaction in the presence of the aforementionedcatalyst to produce an unsaturated carboxylic acid.

[0083] In the production of such an unsaturated carboxylic acid, it ispreferred to employ a starting material gas which contains steam. Insuch a case, as a starting material gas to be supplied to the reactionsystem, a gas mixture comprising a steam-containing alkane, or asteam-containing mixture of alkane and alkene, and an oxygen-containinggas, is usually used. However, the steam-containing alkane, or thesteam-containing mixture of alkane and alkene, and the oxygen-containinggas may be alternately supplied to the reaction system. The steam to beemployed may be present in the form of steam gas in the reaction system,and the manner of its introduction is not particularly limited.

[0084] Further, as a diluting gas, an inert gas such as nitrogen, argonor helium may be supplied. The molar ratio (alkane or mixture of alkaneand alkene):(oxygen):(diluting gas):(H₂O) in the starting material gasis preferably (1):(0.1 to 10):(0 to 20):(0.2 to 70), more preferably(1):(1 to 5.0):(0 to 10):(5 to 40).

[0085] When steam is supplied together with the alkane, or the mixtureof alkane and alkene, as starting material gas, the selectivity for anunsaturated carboxylic acid is distinctly improved, and the unsaturatedcarboxylic acid can be obtained from the alkane, or mixture of alkaneand alkene, in good yield simply by contacting in one stage. However,the conventional technique utilizes a diluting gas such as nitrogen,argon or helium for the purpose of diluting the starting material. Assuch a diluting gas, to adjust the space velocity, the oxygen partialpressure and the steam partial pressure, an inert gas such as nitrogen,argon or helium may be used together with the steam.

[0086] As the starting material alkane it is preferred to employ aC₃₋₈alkane, particularly propane, isobutane or n-butane; morepreferably, propane or isobutane; most preferably, propane. According tothe present invention, from such an alkane, an unsaturated carboxylicacid such as an α,β-unsaturated carboxylic acid can be obtained in goodyield. For example, when propane or isobutane is used as the startingmaterial alkane, acrylic acid or methacrylic acid will be obtained,respectively, in good yield.

[0087] In the present invention, as the starting material mixture ofalkane and alkene, it is preferred to employ a mixture of C₃₋₈alkane andC₃₋₈alkene, particularly propane and propene, isobutane and isobutene orn-butane and n-butene. As the starting material mixture of alkane andalkene, propane and propene or isobutane and isobutene are morepreferred. Most preferred is a mixture of propane and propene. Accordingto the present invention, from such a mixture of an alkane and analkene, an unsaturated carboxylic acid such as an α,β-unsaturatedcarboxylic acid can be obtained in good yield. For example, when propaneand propene or isobutane and isobutene are used as the starting materialmixture of alkane and alkene, acrylic acid or methacrylic acid will beobtained, respectively, in good yield. Preferably, in the mixture ofalkane and alkene, the alkene is present in an amount of at least 0.5%by weight, more preferably at least 1.0% by weight to 95% by weight;most preferably, 3% by weight to 90% by weight.

[0088] As an alternative, an alkanol, such as isobutanol, which willdehydrate under the reaction conditions to form its correspondingalkene, i.e. isobutene, may also be used as a feed to the presentprocess or in conjunction with the previously mentioned feed streams.

[0089] The purity of the starting material alkane is not particularlylimited, and an alkane containing a lower alkane such as methane orethane, air or carbon dioxide, as impurities, may be used without anyparticular problem. Further, the starting material alkane may be amixture of various alkanes. Similarly, the purity of the startingmaterial mixture of alkane and alkene is not particularly limited, and amixture of alkane and alkene containing a lower alkene such as ethene, alower alkane such as methane or ethane, air or carbon dioxide, asimpurities, may be used without any particular problem. Further, thestarting material mixture of alkane and alkene may be a mixture ofvarious alkanes and alkenes.

[0090] There is no limitation on the source of the alkene. It may bepurchased, per se, or in admixture with an alkane and/or otherimpurities. Alternatively, it can be obtained as a by-product of alkaneoxidation. Similarly, there is no limitation on the source of thealkane. It may be purchased, per se, or in admixture with an alkeneand/or other impurities. Moreover, the alkane, regardless of source, andthe alkene, regardless of source, may be blended as desired.

[0091] The detailed mechanism of the oxidation reaction of the presentinvention is not clearly understood, but the oxidation reaction iscarried out by oxygen atoms present in the above promoted mixed metaloxide or by molecular oxygen present in the feed gas. To incorporatemolecular oxygen into the feed gas, such molecular oxygen may be pureoxygen gas. However, it is usually more economical to use anoxygen-containing gas such as air, since purity is not particularlyrequired.

[0092] It is also possible to use only an alkane, or a mixture of alkaneand alkene, substantially in the absence of molecular oxygen for thevapor phase catalytic reaction. In such a case, it is preferred to adopta method wherein a part of the catalyst is appropriately withdrawn fromthe reaction zone from time to time, then sent to an oxidationregenerator, regenerated and then returned to the reaction zone forreuse. As the regeneration method of the catalyst, a method may, forexample, be mentioned which comprises contacting an oxidative gas suchas oxygen, air or nitrogen monoxide with the catalyst in the regeneratorusually at a temperature of from 300° to 600° C.

[0093] The process of the present invention will be described in furtherdetail with respect to a case where propane is used as the startingmaterial alkane and air is used as the oxygen source. The reactionsystem may be a fixed bed system or a fluidized bed system. However,since the reaction is an exothermic reaction, a fluidized bed system maypreferably be employed whereby it is easy to control the reactiontemperature. The proportion of air to be supplied to the reaction systemis important for the selectivity for the resulting acrylic acid, and itis usually at most 25 moles, preferably from 0.2 to 18 moles per mole ofpropane, whereby high selectivity for acrylic acid can be obtained. Thisreaction can be conducted usually under atmospheric pressure, but may beconducted under a slightly elevated pressure or slightly reducedpressure. With respect to other alkanes such as isobutane, or tomixtures of alkanes and alkenes such as propane and propene, thecomposition of the feed gas may be selected in accordance with theconditions for propane.

[0094] Typical reaction conditions for the oxidation of propane orisobutane to acrylic acid or methacrylic acid may be utilized in thepractice of the present invention. The process may be practiced in asingle pass mode (only fresh feed is fed to the reactor) or in a recyclemode (at least a portion of the reactor effluent is returned to thereactor). General conditions for the process of the present inventionare as follows: the reaction temperature can vary from 200° C. to 700°C., but is usually in the range of from 200° C. to 550° C., morepreferably 250° C. to 480° C., most preferably 300° C. to 400° C.; thegas space velocity, SV, in the vapor phase reaction is usually within arange of from 100 to 10,000 hr⁻¹, preferably 300 to 6,000 hr⁻¹, morepreferably 300 to 2,000 hr⁻¹; the average contact time with the catalystcan be from 0.01 to 10 seconds or more, but is usually in the range offrom 0.1 to 10 seconds, preferably from 2 to 6 seconds; the pressure inthe reaction zone usually ranges from 0 to 75 psig, but is preferably nomore than 50 psig. In a single pass mode process, it is preferred thatthe oxygen be supplied from an oxygen-containing gas such as air. Thesingle pass mode process may also be practiced with oxygen addition. Inthe practice of the recycle mode process, oxygen gas by itself is thepreferred source so as to avoid the build up of inert gases in thereaction zone.

[0095] Of course, in the oxidation reaction of the present invention, itis important that the hydrocarbon and oxygen concentrations in the feedgases be maintained at the appropriate levels to minimize or avoidentering a flammable regime within the reaction zone or especially atthe outlet of the reactor zone. Generally, it is preferred that theoutlet oxygen levels be low to both minimize after-burning and,particularly, in the recycle mode of operation, to minimize the amountof oxygen in the recycled gaseous effluent stream. In addition,operation of the reaction at a low temperature (below 450° C.) isextremely attractive because after-burning becomes less of a problemwhich enables the attainment of higher selectivity to the desiredproducts. The catalyst of the present invention operates moreefficiently at the lower temperature range set forth above,significantly reducing the formation of acetic acid and carbon oxides,and increasing selectivity to acrylic acid. As a diluting gas to adjustthe space velocity and the oxygen partial pressure, an inert gas such asnitrogen, argon or helium may be employed.

[0096] When the oxidation reaction of propane, and especially theoxidation reaction of propane and propene, is conducted by the method ofthe present invention, carbon monoxide, carbon dioxide, acetic acid,etc. may be produced as by-products, in addition to acrylic acid.Further, in the method of the present invention, an unsaturated aldehydemay sometimes be formed depending upon the reaction conditions. Forexample, when propane is present in the starting material mixture,acrolein may be formed; and when isobutane is present in the startingmaterial mixture, methacrolein may be formed. In such a case, such anunsaturated aldehyde can be converted to the desired unsaturatedcarboxylic acid by subjecting it again to the vapor phase catalyticoxidation with the promoted mixed metal oxide-containing catalyst of thepresent invention or by subjecting it to a vapor phase catalyticoxidation reaction with a conventional oxidation reaction catalyst foran unsaturated aldehyde.

[0097] As previously noted, the present invention also provides aprocess for producing an unsaturated nitrile, which comprises subjectingan alkane, or a mixture of an alkane and an alkene, and ammonia to avapor phase catalytic oxidation reaction in the presence of theaforementioned catalyst to produce an unsaturated nitrile.

[0098] In the production of such an unsaturated nitrile, as the startingmaterial alkane, it is preferred to employ a C₃₋₈alkane such as propane,butane, isobutane, pentane, hexane and heptane. However, in view of theindustrial application of nitrites to be produced, it is preferred toemploy a lower alkane having 3 or 4 carbon atoms, particularly propaneand isobutane.

[0099] Similarly, as the starting material mixture of alkane and alkene,it is possible to employ a mixture of C₃₋₈alkane and C₃₋₈alkene such aspropane and propene, butane and butene, isobutane and isobutene, pentaneand pentene, hexane and hexene, and heptane and heptene. However, inview of the industrial application of nitrites to be produced, it ismore preferred to employ a mixture of a lower alkane having 3 or 4carbon atoms and a lower alkene having 3 or 4 carbon atoms, particularlypropane and propene or isobutane and isobutene. Preferably, in themixture of alkane and alkene, the alkene is present in an amount of atleast 0.5% by weight, more preferably at least 1.0% by weight to 95% byweight, most preferably 3% by weight to 90% by weight.

[0100] The purity of the starting material alkane is not particularlylimited, and an alkane containing a lower alkane such as methane orethane, air or carbon dioxide, as impurities, may be used without anyparticular problem. Further, the starting material alkane may be amixture of various alkanes. Similarly, the purity of the startingmaterial mixture of alkane and alkene is not particularly limited, and amixture of alkane and alkene containing a lower alkene such as ethene, alower alkane such as methane or ethane, air or carbon dioxide, asimpurities, may be used without any particular problem. Further, thestarting material mixture of alkane and alkene may be a mixture ofvarious alkanes and alkenes.

[0101] There is no limitation on the source of the alkene. It may bepurchased, per se, or in admixture with an alkane and/or otherimpurities. Alternatively, it can be obtained as a by-product of alkaneoxidation. Similarly, there is no limitation on the source of thealkane. It may be purchased, per se, or in admixture with an alkeneand/or other impurities. Moreover, the alkane, regardless of source, andthe alkene, regardless of source, may be blended as desired.

[0102] The detailed mechanism of the ammoxidation reaction of thisaspect of the present invention is not clearly understood. However, theoxidation reaction is conducted by the oxygen atoms present in the abovepromoted mixed metal oxide or by the molecular oxygen in the feed gas.When molecular oxygen is incorporated in the feed gas, the oxygen may bepure oxygen gas. However, since high purity is not required, it isusually economical to use an oxygen-containing gas such as air.

[0103] As the feed gas, it is possible to use a gas mixture comprisingan alkane, or a mixture of an alkane and an alkene, ammonia and anoxygen-containing gas, However, a gas mixture comprising an alkane or amixture of an alkane and an alkene and ammonia, and an oxygen-containinggas may be supplied alternately.

[0104] When the gas phase catalytic reaction is conducted using analkane, or a mixture of an alkane and an alkene, and ammoniasubstantially free from molecular oxygen, as the feed gas, it isadvisable to employ a method wherein a part of the catalyst isperiodically withdrawn and sent to an oxidation regenerator forregeneration, and the regenerated catalyst is returned to the reactionzone. As a method for regenerating the catalyst, a method may bementioned wherein an oxidizing gas such as oxygen, air or nitrogenmonoxide is permitted to flow through the catalyst in the regeneratorusually at a temperature of from 300° C. to 600° C.

[0105] These aspects of the present invention will be described infurther detail with respect to a case where propane is used as thestarting material alkane and air is used as the oxygen source. Theproportion of air to be supplied for the reaction is important withrespect to the selectivity for the resulting acrylonitrile. Namely, highselectivity for acrylonitrile is obtained when air is supplied within arange of at most 25 moles, particularly 1 to 15 moles, per mole of thepropane. The proportion of ammonia to be supplied for the reaction ispreferably within a range of from 0.2 to 5 moles, particularly from 0.5to 3 moles, per mole of propane. This reaction may usually be conductedunder atmospheric pressure, but may be conducted under a slightlyincreased pressure or a slightly reduced pressure. With respect to otheralkanes such as isobutane, or to mixtures of alkanes and alkenes such aspropane and propene, the composition of the feed gas may be selected inaccordance with the conditions for propane.

[0106] The processes of these aspects of the present invention may beconducted at a temperature of, for example, from 250° C. to 480° C. Morepreferably, the temperature is from 300° C. to 400° C. The gas spacevelocity, SV, in the gas phase reaction is usually within the range offrom 100 to 10,000 hr⁻¹, preferably from 300 to 6,000 hr⁻¹, morepreferably from 300 to 2,000 hr⁻¹. As a diluent gas, for adjusting thespace velocity and the oxygen partial pressure, an inert gas such asnitrogen, argon or helium can be employed. When ammoxidation of propaneis conducted by the method of the present invention, in addition toacrylonitrile, carbon monoxide, carbon dioxide, acetonitrile,hydrocyanic acid and acrolein may form as by-products.

EXAMPLES Catalyst Preparation Comparative Example 1

[0107] Ammonium heptamolybdate (49.7 g), ammonium metavanadate (9.8 g)and telluric acid (14.9 g) were dissolved in water at 80° C., and thissolution was allowed to cool to 65° C. Oxalic acid dihydrate (5.5 g) wasdissolved in a 6.5% w/w solution of niobium oxalate in water (289.9 g),and the resulting niobium oxalate solution, having a temperature of 25°C., was added to the first solution, having a temperature of 65° C. Themixture was dried in a rotary evaporator, and then overnight undervacuum. The dried solid was calcined 1 hour at 275° C. in air, and then2 hours at 600° C. in argon.

Example 1

[0108] The catalyst was prepared using the same procedure as ComparativeExample 1, except that the first solution was maintained at 80° C. andthe niobium oxalate solution, having a temperature of 25° C., was addedto the first solution, having a temperature of 80° C.

Example 2

[0109] The catalyst was prepared using the same procedure as ComparativeExample 1, except that the first solution was heated to 100° C., theniobium oxalate solution was heated to 100° C., and the niobium oxalatesolution, having a temperature of 100° C., was added to the firstsolution, having a temperature of 100° C.

Comparative Example 2

[0110] The catalyst was prepared using the same procedure as ComparativeExample 1, except that the first solution was allowed to cool to 25° C.,and the niobium oxalate solution , having a temperature of 25° C., wasadded to the first solution, having a temperature of 25° C.

[0111] Evaluation and Results

[0112] Catalysts were evaluated for the partial oxidation of propane toacrylic acid. The results are shown in Table 1. TABLE 1 Acrylic AcrylicTemp. Reaction Propane Acid Acid Of Mixing Temp. Conv. Selectivity YieldExample (° C.) (° C.) (%) (%) (%) Comp. Ex. 65/25 364 68.4 54.2 37.0 1Ex. 1 80/25 361 79.5 62.3 49.6 Ex. 2 100/100 350 74.2 58.0 43.1 Comp.Ex. 25/25 380 63.5 57.6 36.6 2

What is claimed is:
 1. A process for producing an improved catalystcomprising a mixed metal oxide having the empirical formulaMo_(a)V_(b)N_(c)X_(d)Z_(e)O_(f) wherein N is at least one elementselected from the group consisting of Te and Sb, wherein X is at leastone element selected from the group consisting of Nb, Ta, Ti, W, Se, Al,Zr, Cr, Mn, Fe, Ru, Co, Rh, Ni, Pt, Bi, B, In, As, Ge, Sn, Li, Na, K,Rb, Cs, Fr, Be, Mg, Ca, Sr, Ba, Ra, Hf, Pb, P, Pm, Eu, Gd, Dy, Ho, Er,Tm, Yb and Lu, wherein Z is at least one element selected from the groupconsisting of Au, Ag, Re, Pr, Zn, Ga, Pd, Ir, Nd, Y, Sm, Tb, Br, Cu, Sc,Cl, F and I, and wherein, when a=1, b=0.01 to 1.0, c=0.01 to 1.0, d=0.01to 1.0, e=0 to 0.1 and f is dependent on the oxidation state of theother elements, comprising: (a) admixing compounds of elements Mo, V, N,X and Z and at least one solvent at a temperature greater than or equalto 45° C. to form an admixture, wherein Mo, V, N, X and Z are present insuch amounts that the atomic ratio of Mo:V:N:X:Z is a:b:c:d:e, and, whena=1, b=0.01 to 1.0, c=0.1 to 1.0, d=0.01 to 1.0 and e=0 to 0.1; (b)removing said at least one solvent from the so-formed admixture to forma catalyst precursor; and (c) calcining said catalyst precursor toobtain said mixed metal oxide.
 2. The process according to claim 1,wherein e=0.001 to 0.1.
 3. The process according to claim 1, whereinsaid catalyst precursor is calcined in two stages, in a first stage,calcination is effected under an oxidizing atmosphere, and, in a secondstage, calcination is effected under a non-oxidizing atmosphere.
 4. Thecatalyst produced by the process according to claim
 1. 5. A process forproducing an unsaturated carboxylic acid which comprises subjecting analkane, or a mixture of an alkane and an alkene, to a vapor phasecatalytic oxidation reaction in the presence of a catalyst comprising amixed metal oxide having the empirical formulaMo_(a)V_(b)N_(c)X_(d)Z_(e)O_(f) wherein N is at least one elementselected from the group consisting of Te and Sb, wherein X is at leastone element selected from the group consisting of Nb, Ta, Ti, W, Se, Al,Zr, Cr, Mn, Fe, Ru, Co, Rh, Ni, Pt, Bi, B, In, As, Ge, Sn, Li, Na, K,Rb, Cs, Fr, Be, Mg, Ca, Sr, Ba, Ra, Hf, Pb, P, Pm, Eu, Gd, Dy, Ho, Er,Tm, Yb and Lu, wherein Z is at least one element selected from the groupconsisting of Au, Ag, Re, Pr, Zn, Ga, Pd, Ir, Nd, Y, Sm, Tb, Br, Cu, Sc,Cl, F and I, and wherein, when a=1, b=0.01 to 1.0, c=0.01 to 1.0, d=0.01to 1.0, e=0 to 0.1 and f is dependent on the oxidation state of theother elements, prepared by the process comprising: (a) admixingcompounds of elements Mo, V, N, X and Z and at least one solvent at atemperature greater than or equal to 45° C. to form an admixture,wherein Mo, V, N, X and Z are present in such amounts that the atomicratio of Mo:V:N:X:Z is a:b:c:d:e, and, when a=1, b=0.01 to 1.0, c=0.1 to1.0, d=0.01 to 1.0 and e=0 to 0.1; (b) removing said at least onesolvent from the so-formed admixture to form a catalyst precursor; and(c) calcining said catalyst precursor to obtain said mixed metal oxide.6. A process for producing an unsaturated nitrile which comprisessubjecting an alkane, or a mixture of an alkane and an alkene, andammonia to a vapor phase catalytic oxidation reaction in the presence ofa catalyst comprising a mixed metal oxide having the empirical formulaMo_(a)V_(b)N_(c)X_(d)Z_(e)O_(f) wherein N is at least one elementselected from the group consisting of Te and Sb, wherein X is at leastone element selected from the group consisting of Nb, Ta, Ti, W, Se, Al,Zr, Cr, Mn, Fe, Ru, Co, Rh, Ni, Pt, Bi, B, In, As, Ge, Sn, Li, Na, K,Rb, Cs, Fr, Be, Mg, Ca, Sr, Ba, Ra, Hf, Pb, P, Pm, Eu, Gd, Dy, Ho, Er,Tm, Yb and Lu, wherein Z is at least one element selected from the groupconsisting of Au, Ag, Re, Pr, Zn, Ga, Pd, Ir, Nd, Y, Sm, Tb, Br, Cu, Sc,Cl, F and I, and wherein, when a=1, b=0.01 to 1.0, c=0.01 to 1.0, d=0.01to 1.0, e=0 to 0.1 and f is dependent on the oxidation state of theother elements, prepared by the process comprising: (a) admixingcompounds of elements Mo, V, N, X and Z and at least one solvent at atemperature greater than or equal to 60° C. to form an admixture,wherein Mo, V, N, X and Z are present in such amounts that the atomicratio of Mo:V:N:X:Z is a:b:c:d:e, and, when a=1, b=0.01 to 1.0, c=0.1 to1.0, d=0.01 to 1.0 and e=0 to 0.1; (b) removing said at least onesolvent from the so-formed admixture to form a catalyst precursor; and(c) calcining said catalyst precursor to obtain said mixed metal oxide.